Industrialization and population growth increases the petroleum demand. The reserved fossil fuel is rapidly diminishing that increases crude oil prices affecting growth of industrialization. This has motivated scientists to look for alternative energy sources which are renewable, economical and environmentally friendly. As an example, there is an increased interest in converting abundantly available biomass to produce valuable chemicals and fuels.1 
Biodiesel is one of the potential environmentally friendly substitutes for petroleum based diesel fuel. The transesterification of vegetable oil is an example of a process to produce biodiesel which has already been commercialized.1,2 The global market for biodiesel is estimated to reach 180 million tons by 2016 and to grow at the rate of 42% per year.3,4 In the biodiesel production process, glycerol is produced in the amount of 10% of the products of the process.5 Glycerol has been considered as one of the top 12 building block chemicals by the U.S. Department of Energy and can replace some of the chemicals derived from fossil fuel.4,5,6,7 The large amount of glycerol produced by the biodiesel industry is currently not completely utilized by chemical industries leading to a decrease in its price. Therefore, production of a commercially valuable product from glycerol using an economically feasible process is desirable.4 
Several chemicals can be derived from glycerol, including 1-2 and 1,3-propanediols, acrolein, acrylic acid, epichlorohydrin, 3-hydroxypropionic acid and other specialty chemicals. Among these chemicals, the production of 1,2- and 1,3-propanediol via glycerol hydrogenolyis has attracted significant commercial interest (FIG. 1). 1,2-Propandiol, also known as propylene glycol, is a major commodity chemical and has seen a 4% market growth every year.4 Propylene glycol has several commercial applications, for example, as antifreeze, coolant, solvent and extractant, deicing agent, precursor in pharmaceuticals, cosmetics, animal food and tobacco industries, petroleum production, sugar refining, paper making, toiletries, liquid detergent, alkyl resins, printing inks, plasticizers, and hydraulic break fluids.8,9,10 
Propylene glycol is currently produced from petroleum derivatives such as propylene oxide and chlorohydrin by chemical routes.11,12,13 The sharp increase in oil price and the declining petroleum resource has made this route expensive. Therefore the production of propylene glycol from renewable resources such as glycerol has attracted much attention. Che et al.14 used [Rh(CO)2(acac)] and tungstenic acid as a homogeneous catalyst for hydrogenolysis of aqueous glycerol with syngas at 30 MPa and 200° C. and reported 20% and 30% yield of 1,3-propandiol and 1,2-propandiol, respectively. Drent and Jager15 used a homogeneous palladium complex and methanesulfonic acid and reported 22% yield of propylene glycol. Schlaf et al.16 used a homogeneous ruthenium complex as catalyst for dehydroxylation of glycerol in sulfolane at 5.2 MPa and 110° C. and obtained low yield of propylene glycol. Various heterogeneous catalysts, mainly Cu based mixed oxides and supported noble metals have been used for hydrogenolysis of glycerol. Cu is known to suppress C—C bond cleavage and for C—O bond breaking in glycerol to produce propylene glycol.17,18 Mane et al.19 prepared Cu—Al nanocatalysts by a simultaneous co-precipitation method and obtained 91% selectivity to propylene glycol at 220° C. and 7 MPa H2 pressure in 5 h. Some papers also mentioned the use of bimetallic catalysts such as Cu/C, Cu—Pt, and Cu—Ru.20,21 Roy et al.1 studied the conversion of glycerol to propylene glycol using a mixture of 5 wt. % Ru/Al2O3 and 5 wt. % Pt/Al2O3 catalysts in varying amounts utilizing in situ generated hydrogen and reported 47.2% selectivity of propylene glycol at 50% conversion of glycerol. Schmidt et al.22 used a Raney-Cu based catalyst in trickle bed mode and obtained 94% selectivity to propylene glycol and 100% glycerol conversion. Werpy et al.23 reported glycerol hydrogenolysis over Ni/Re catalyst under 8.2 MPa H2 and 230° C. in 4 h which led to 44% 1,2-propandiol yield. The use of organic supported catalysts, including carbon supported Ru, Pt, and bimetallic Pt—Ru and Al—Ru catalyst has also been reported.24,25 Also, the Cu/ZnO based catalysts have been reported to give a high catalytic performance for the glycerol dehydroxylation reaction to propylene glycol under mild reaction conditions.26,27 A Cu/Cr2O3 catalyst was reported to produce propylene glycol from glycerol.28,29 Amberlyst-15 was used in addition with Ru to induce external acidity in the catalyst and an improvement in glycerol conversion was documented.30,31 Vasiliadou and Lemonidou4 reported that the total acidity of catalyst (induced either by support and/or by the metal precursor) strongly affects the glycerol conversion. It is reported that ZnO acts as a reservoir for atomic hydrogen and promotes hydrogen spillover for the reaction which can increase the activity of the catalyst.32-33 Raju et al.34 reported that a ZrO2 based catalyst is responsible for acetol production from glycerol. Several reports support the acidic nature of ZrO2.35,36,37,38,39 
Despite the report of several investigations, the heterogeneous catalyzed glycerol hydrogenolysis process still has several drawbacks which limit its scale up to pilot plant level. These problems include the use of dilute glycerol solution (10-30%), high temperatures (300-350° C.), high pressures (10-30 MPa), poor catalyst reusability, catalyst leaching, lower glycerol conversion and/or lower propylene glycol selectivity. The selective hydrogenolysis of glycerol to propylene glycol requires the preferential cleavage, by hydrogen, of the C—O bond over the C—C bond in the glycerol molecule.40 